The present invention relates to a subscriber unit for use in a telephone system. The subscriber unit of the invention is particularly useful with respect to the time-division-multiplex system described in copending Pat. application Ser. No. 517,533, filed Oct. 24, 1974, by the same applicant as the present application. The present invention is therefore described below particularly with respect to that application, but it will be appreciated that it, or certain of its features, could be advantageously used in other systems as well.
One of the most important components of electronic telephone exchanges is the line circuit interface unit which interfaces the electronic part of the exchange with the central-battery subscriber's set. The line circuit supplies the direct current needed for the subscriber's set for signalling and also for driving the subscriber's microphone. In addition, the line circuit provides the relatively high ringing voltage, and also protects against excess voltage surges. Further included in the line circuit are the off-hook and dialling detectors and the transfer detector.
The line circuit is usually connected to the exchange through a line transformer whose coil on the line side is split into two equal parts, the direct current voltage being connected through resistors, chokes, or a relay, between the taps of the two parts of the transformer coil. Such a circuit isolates the power supply, thereby enabling the connection to it of many line circuits. In addition, the circuit enables the detection of the subscriber's signalling.
In such a system, the line wires can be easily unbalanced by any unbalance of the transformer parts, relay coil, or resistors, resulting in increased cross-talk. Further, the transformer has to pass, without saturating, 30-100 mA direct current in accordance with the line's length. This requires a transformer of substantial size to prevent saturation, even though the power delivered by the transformer is only about 1 mW. Also, ballasts may be required to guard the microphone of the subscriber's set particularly on short lines.
An object of the present invention is to provide a line circuit which provides advantages in one or more of the above respects.